This invention relates to the reduction of the NO.sub.x content in the flue gas of single stage combustion boilers equipped with plural burners fired by liquid or gaseous fuel.
Nitric oxides (NO.sub.x) are produced, during the combustion of fossil fuels, by oxidation of NO, which is harmless in itself. They may originate in the atmospheric nitrogen or in the nitrogen contained in the fuel. Since, in greater concentration, nitric oxides are noxious to animal and human life, every effort is made to keep their content in the air reduced below a definite maximum value.
For reducing the NO.sub.x content, it is well known to lower the flame temperature and/or to adjust a near stoichiometric air-to-fuel ratio. One means for lowering the flame temperature is flue gas recirculation, during which cooler flue gas is introduced into the combustion space for cooling the flame. The formation of NO.sub.x, however, may also be limited by reducing the amount of excess air during combustion. This is provided in two-stage combustion where, in the first stage, the combustion takes place with a deficiency of air. Thereby, the flame temperature is limited and, between the first and the second stage, the gases are cooled before the combustion is terminated.
Although both of these methods reduce the NO.sub.x content in the flue gas, they have other disadvantages. They lead to an entrainment of unburned components and diminish the efficiency of the plant, and also require higher expenditures for construction and control.
Up to date, constructional measures have not led to an effective reduction of the NO.sub.x content. According to a study published some time ago, a particular positioning of the burners (corner or front firing) has little influce on the NO.sub.x content. Even though smaller boilers show a smaller NO.sub.x content in the flue gases than large ones, the content of NO.sub.x in the waste gases of small boilers is more dangerous since it does not pass into the atmosphere through high chimneys.